Well, for souvenirs we usually get 1 t-shirt/sweatshirt per person and each of the kids gets to pick out one toy (usually a med. sized stuffed character doll). In the past, we've also gotten one of the light spinners for DS3. But this year, I've purchased some glow-in-the-dark necklaces to take with us instead of buying yet another light spinner -- check your dollar store for glow in the dark items. I've also purchased some Disney stickers to take as rewards when the kids are being good.
My biggest non-spending trip is to stay out of the stores at the parks. Instead, concentrate on doing things that are already paid for like attractions and shows.
Make a grocery run or take snacks from home. I read a hint from another poster who made up a big bag of snacks and let the kids pick each morning which snacks to take to the parks that day (some healthy and some not so healthy). That works well for us -- I pack bottled water for everyone, juice boxes for the kids, granola bars, trail mix, animal crackers, and tootsie pops. If we see a snack that we really want, we get one or two and share.
Since we don't go in the stores at the parks, we make a trip to the World of
Disney Store toward the end of our trip to pick out a t-shirt for each of us and let the kids pick out one toy. We don't go hog-wild -- on our last visit, there was a cute sweatshirt that I liked that was $70 and I said, "Well, I won't spend $70 for a sweatshirt at home and I'm not spending $70 for a sweatshirt here." I think I found a t-shirt that was just as nice for $15.
We also don't generally buy anything for anyone else -- not postcards or t-shirts or anything.
There is also supposed to be a store at DTD where everything is less than $10. That might be a good place to look for a deal.
So, for souvenirs, we'll probably spend $150-$200 for the four of us. Maybe another $50-$100 for snacks. So, $300 tops for spending money. (Though I have read a post recently from someone who plans to spend NO MONEY on souvenirs. Don't know that I could do that!)
I hope this helps!